Biology module 2.5

• Biological membranes


  All cells and organelles are surrounded by a partially permeable membrane composed of a sea of phospholipids with protein molecules between the phospholipid molecules
• Biological membranes


  • The main function is controlling the movement of substances in and out of the cell/organelle
  • They also contain receptors for other molecules such as hormones
  • They enable adjacent cells to stick together
  • The fluidity of the membrane and the mosaic arrangement of the protein give the structure of the membrane its name – fluid mosaic model
• Movement of molecules through cell membrane

  1. Diffusion

  2. Facilitated diffusion

  3. Osmosis

  4. Active transport

  5. Exocytosis

  6. Endocytosis
• Diffusion


  Passive movement of small, non-polar lipid soluble molecules such as carbon dioxide and oxygen from an area of high concentration to an area of low concentration through the phospholipid bilayer
• Facilitated diffusion


  Requires a channel protein in the cell membrane to transport polar molecules, charged and water soluble molecules across the membrane
• Osmosis


  Diffusion of water molecules from an area of low solute concentration to an area of high solute concentration through a partially permeable membrane
• Osmosis


  Diffusion of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane
• Active transport


  Can transport all types of molecules through carrier proteins from an area of low concentration to an area of high concentration, but requires energy in the form of ATP
• Exocytosis


  Transport of large particles and bulk transport, where vesicles containing large particles are fused with the cell surface membrane
• Endocytosis


  Transport of large particles and bulk transport, where particles are enclosed in vesicles made from the cell surface membrane and transported into the cell
• The rate of gas exchange by diffusion


  Becomes more rapid as surface area increases, diffusion distance decreases, and diffusion gradient becomes more steep
• Membranes are vital structures found in all cells
• Cell surface membrane


  Creates an enclosed space separating the internal cell environment from the external environment
• Intracellular membranes


  Form compartments within the cell, such as organelles (including the nucleus, mitochondria and RER) and vacuoles
• Membranes


  • Not only separate different areas but also control the exchange of materials passing through them; they are partially permeable
  • Form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles
• Substances can cross membranes by


  Diffusion, facilitated diffusion, osmosis and active transport
• Membranes


  Play a role in cell signalling by acting as an interface for communication between cells
• Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane